A MRAM is a promising nonvolatile memory from a viewpoint of high integration and a high-speed operation. In the MRAM, a magnetoresistance element that exhibits a “magnetoresistance effect” such as a TMR (Tunnel MagnetoResistance) effect is utilized. In the magnetoresistance element, for example, a MTJ (Magnetic Tunnel Junction) in which a tunnel barrier layer is sandwiched by two ferromagnetic layers is formed. The two ferromagnetic layers include a pinned layer whose magnetization orientation is fixed and a free layer whose magnetization orientation is reversible.
It is known that a resistance value (R+ΔR) of the MTJ when the magnetization orientations of the pinned layer and the free layer are “anti-parallel” to each other becomes larger than a resistance value (R) when the magnetization orientations are “parallel” to each other, due to the magnetoresistance effect. The MRAM uses the magnetoresistance element having the MTJ as a memory cell and nonvolatilely stores data by utilizing the change in the resistance value. Data writing to the memory cell is performed by switching the magnetization orientation of the free layer.
Conventionally known methods of data writing to the MRAM include an “asteroid method” and a “toggle method”. According to these write methods, a magnetic switching field necessary for switching the magnetization of the free layer increases in substantially inverse proportion to the size of the memory cell. That is to say, a write current tends to increase with the miniaturization of the memory cell.
As a write method capable of suppressing the increase in the write current with the miniaturization, there is proposed a “spin transfer method”. For example, refer to Japanese Laid-Open Patent Application JP-2005-93488 or “Yagami and Suzuki, Research Trends in Spin Transfer Magnetization Switching, Journal of The Magnetics Society of Japan, Vol. 28, No. 9, 2004”. According to the spin transfer method, a spin-polarized current is injected to a ferromagnetic conductor, and direct interaction between spin of conduction electrons of the current and magnetic moment of the conductor causes the magnetization to be switched (hereinafter referred to as “spin transfer magnetization switching”). The spin transfer magnetization switching will be outlined below with reference to FIG. 1.
In FIG. 1, a magnetoresistance element is provided with a free layer 101, a pinned layer 103 and a tunnel barrier layer 102 that is a nonmagnetic layer sandwiched between the free layer 101 and the pinned layer 103. Here, the pinned layer 103, whose magnetization orientation is fixed, is so formed as to be thicker than the free layer 101 and serves as a spin filter, i.e. a mechanism for generating the spin-polarized current. A state in which the magnetization orientations of the free layer 101 and the pinned layer 103 are parallel to each other is related to data “0”, while a state in which they are anti-parallel to each other is related to data “1”.
The spin transfer magnetization switching shown in FIG. 1 is achieved by a CPP (Current Perpendicular to Plane) method, where a write current is injected in a direction perpendicular to the film plane. More specifically, the current is flowed from the pinned layer 103 to the free layer 101 in a transition from data “0” to data “1”. In this case, electrons having the same spin state as that of the pinned layer 103 being the spin filter move from the free layer 101 to the pinned layer 103. As a result of the spin transfer (transfer of spin angular momentum) effect, the magnetization of the free layer 101 is switched. On the other hand, the current direction is reversed and the current is flowed from the free layer 101 to the pinned layer 103 in a transition from data “1” to data “0”. In this case, electrons having the same spin state as that of the pinned layer 103 being the spin filter move from the pinned layer 103 to the free layer 101. As a result of the spin transfer effect, the magnetization of the free layer 101 is switched.
In this manner, the data writing is performed by transferring the spin electrons in the spin transfer magnetization switching. It is possible to set the magnetization orientation of the free layer 101 depending on the direction of the spin-polarized current perpendicular to the film plane. Here, it is known that the threshold value of the writing (magnetization switching) depends on current density. Therefore, the write current necessary for the magnetization switching decreases with the reduction of the size of the memory cell. Since the write current is decreased with the miniaturization of the memory cell, the spin transfer magnetization switching is important in realizing a large-capacity MRAM.
As a related technique, domain wall motion by the spin transfer in a magnetic substance is described in Japanese Laid-Open Patent Application JP-2006-73930 and Japanese Laid-Open Patent Application JP-2005-191032.
A magnetic memory element described in the Japanese Laid-Open Patent Application JP-2006-73930 is provided with a first magnetic layer, an intermediate layer, and a second magnetic layer. Information is recorded on the basis of a relationship between a magnetization orientation of the first magnetic layer and a magnetization orientation of the second magnetic layer. Here, magnetic domains having magnetizations anti-parallel to each other and a domain wall separating them are steadily formed in at least one of the magnetic layers. By moving the domain wall in the magnetic layer, positions of the adjacent magnetic domains are controlled and information is recorded.
A magnetic storage device described in the Japanese Laid-Open Patent Application JP-2005-191032 is provided with a magnetization fixed layer whose magnetization is fixed, a tunnel insulating layer laminated on the magnetization fixed layer, and a magnetization free layer laminated on the tunnel insulating layer. The magnetization free layer has a connector section overlapping with the tunnel insulating layer and the magnetization fixed layer, constricted sections adjacent to both ends of the connector section, and a pair of magnetization fixed sections respectively formed adjacent to the constricted sections. The magnetization fixed sections are respectively provided with fixed magnetizations whose directions are opposite to each other. The magnetic storage device is further provided with a pair of magnetic information writing terminals which are electrically connected to the pair of magnetization fixed sections. By using the pair of magnetic information writing terminals, a write current penetrating through the connector section, the pair of constricted sections and the pair of magnetization fixed sections of the magnetization free layer is flowed.
Furthermore, the domain wall motion in a magnetic substance is described also in Japanese Laid-Open Patent Application JP-2005-150303, “Yamaguchi et al., PRL, Vol. 92, pp. 077205-1, 2004”, and others.